X-ray tube



M. MORRISON July 5, 1932.

X-RAY TUBE Original Filed Oct. 18, 1925 w m. L, 5 b m/ ,4 8) 2.

Patented July 5, 1932 UNITED STATES PATENT OFFICE MONTFORD MORRISON, OF CHICAGO, ILLINOIS, ASSIGNOR, BY'MESNE ASSIGNMENTS, TO WESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA X-RAY TUBE Original application filed October 18, 1923, Serial No. 669,315. Divided and this application filed January 4, 1924. Serial No. 684,336.

This application is a division of my copending application, Serial No. 669,815, filed October 18, 1923, transformers and transformerldevices. V

This invention relates to evacuated de vices and more particularly to 'X-ray tubes and related devices.

An object of my invention is the provision of X-ray tubes especially adapted for small dental outfits using high voltage.

Another object of my invention is the provision of an X-ray tube adapted to be insulated by electrostatic field control and cooled by means of air, without the necessity of employing oil for such purposes.

A further object of my invention is the provision of an X-ray tube adapted to be operated on high voltage and designed to be insulated by electrostatic field control and cooled by air, whereby lightness in weight is attained.

A still further object of my invention is the provision of an X-ray tube in which an electron-stream is theremionically generated and flows to the anode or target substantially along the longitudinal axis of the tube and the main beam of X-rays generated flows back along the same axis, through the center of the electron source and out of the tube.

Other objects and advantages of the invention will become apparent as the description proceeds.

X-ray apparatus, especially adapted for dental use, have been constructed in which a high voltage transformer and the X-ray tube are both contained in a small oil-filled tank which is suspended on a'wall bracket. The

entire outfit is made small enough to bereadily placed in various positions necessary for dental X-ray work.

This type of construction entirely obviates P any high tension leads, since all high voltage conductors are contained in a small tank. The disadvantage of such a construction lies in the fact that the tank is filled with oil and it is, therefore, more or less troublesome to replace damaged tubes and more or less dangerous if such a tube breaks while in service.

In my copending application, Serial No.

669,315, filed October 18, 1923, transformer and transformer devices, I have disclosed an outfit of such a type in which the use of oil for insulation and cooling of the X-ray tube is dispensed with and the tube is, instead, insulated by electrostatic field control and cooled by air with the result that it is somewhat lighter in weight than such an oilcooled outfit and because of my special design, is also about the same size.

The present application, which is a division of said copending application, is concerned conveniently connected and disconnected from any auxiliary apparatus, such as a high voltage transformer, with which it is to be used.

When in operation, the tube is preferably supported by said anode screw-plug and the axis of said tube and anode is preferably normally approximately vertical with the anode extending downwardly toward the main substantially-spheroidal portion of the tube from said anode extension, but it will be appreciated that the tube may be used in any position whatever and in the parent application, means have been provided for such positioning of the tube. The anode is preferably rovided with a convex or substantially hemispheroidal end, the tip or working face thereof being formed of some refractory metal of high atomic weight, such as thorium, for example.

Extending from the opposite end of the tube, there is preferably a filamentary cathode adapted to be heated to an electronemitting temperature by a pair of leading-in conductors extending through the main portion of the tube envelope. The conductors are formed heavy enough to support said filamentary cathode and a focusing device surrounding the same.

My invention will better be understood by referring to the accompanying drawing, in which the single figure illustrates a preferred form of my X-ray tube,'shown partly in section and illustrated diagrammatically as being connected to a suitable source of electrical energy and suitable transformers for the operation thereof.

In my copending application before referred to, of which this case is a division, I have described and claimed a preferred form of transformer and casing therefor to be used as part of a dental X-ray outfit. This apnlication is concerned primarily with the X-ray tube for such an outfit and I have, therefore, only illustrated said tube in detail and the associated apparatus is illustrated diagrammatically.

It is obvious that my tube is not restricted for use with dental outfits, but may be used for any purpose, especially where the space for the installation of such a tube is more or less limited, and where the practicability or convenience of such an outfit is advantageous or desirable.

Referring to the drawing, a preferred form of envelope for my X-ray tube is represented at 1, the said envelope being preferably constructed with a main substantially spheroidal portion 2 and an auxiliary substantially cylindrical extension therefrom at 3., The cylindrical extension is preferably provided with a screw-plug 4i adapted to be inserted in a receptacle or socket 5, similar to those in which ordinary incandescent electric lamps are operated. Such a receptacle may form part of a casing and enclosed transformer used in connection with my X-ray tube.

My X-ray tube 1 is novel in construction and related to several of my copending applications, specifically referred to in my aforementioned application. It is substantially symmetrical in design about its longitudinal axis, which is preferably in a generally upright position, when in normal operation, with the extension 3 extending in an upward direction therefrom. Although such a position may be considered as a normal operating position, it is obvious that when my tube is employed as part of a dental outfit, means are preferably provided whereby the same may be moved, as part of said outfit, to any desired position, so as to be conveniently utilized for directing the X-rays generated in a desired path.

Into said extension 3 is preferably sealed an anode or target 6 formed of good heat conducting material, such as copper, and having a substantially spheroidal or convex end 7, Into this end is preferably set a convex tip or working face 8, which is preferably conspheroidal and toroidal geometrical construc tion and a conspicuous absence of points and edges, where said points and edges would influence the electrical field distribution without the focusing device, is preferably employed. The focusing device is provided with a substantially central opening 12 permitting a conically divergent beam of X-rays to pass therethrough and cover the volumetric space represented by the angle 13. The opening in the focusing device and through which the divergent X-rays pass, therefore, serves simultaneously as a diaphragming device.

The focusing device 11 is preferably fixed in position by a plurality of rods or the like 14 and 15, sealed into glass or insulating parts 16 and 17, as is common in the manufacture of evacuated devices, such as lamps. Into glass part 16 is pressed a leading-in conductor 18 which is formed into one or more convolutions, or connected to a filamentary cathode formed into one or more convolutions, at 19. A cathode 19, substantially circular in plan, is illustrated in the drawing, although it is obvious that other forms, consisting of one or more convolutions, may be employed. The said convolution or convolutions is or are of refractory metal adapted to be incandesced without substantial deterioration and are preferably constructed of thorium or other material of good electron-emitting properties, thereby adapting the cathode as a thermionic source of electrons. The other end of the electron-emitting portion 19 is conneeted to or continued as leading-in conductor 21, which is pressed into glass or insulating part 17 and serves as the remaining lead for the thermionic electron source.

Conductors 18 and 21 are sealed into the glass at 22 and 23 respectively. The ends of said conductors are preferably supplied with contact buttons 24: and 25 respectively, which by means of corresponding contacts 26 and 27 respectively, and insulated lead wires 28 and 29 therefrom, serve as means for conducting low voltage electrical current into the bulb of the X-ray tube and supplies heating current for the thermionic cathode 19. Lead wires 28 and 29 are preferably connected through switch 31 to the secondary 32 of a suitable heating transformer 33, the primary of which is represented at 34. The conductor 28 may serve as a common connection for the low voltage winding of high voltage transformer, 35, and the thermionic filament of the X-ray tube. Said high voltage transformer as shown, comprises primary winding 36 and secondary winding 37. The primary winding 36 is preferably connected through lead 38, switch 39 and common lead 28 to alternating current generator 41, or other suitable source of alternating current.

The secondary winding 37 of the high voltage transformer 35, is preferably connected to the anode 6 by lead 42 and to the cathode by the common lead 28 through connection 43. As the cathode end of the X-ray tube is preferably grounded, for example, at 44, it will be apparent that the only portion, of the apparatus shown, at high voltage is the high voltage transformer secondary winding 37 the anode 6 of the tube and the connection 42 therebetween. These portions of the apparatus are away from the patient and operator and may be securely enclosed and insulated, so as to avoid danger in using the device.

The focusing device 11 is preferably constructed of a refractory metal of high atomic weight, such as thorium or the like, and the focal spot 8 on the anode 6 may so project into the spheroidal cavity of the said focusing device, as to substantially prevent the escape of X-rays from the said cavity, except through the opening 12 along the conical space represented at 13. In this way, my focusing device, which is preferably formed of material substantially opaque to X-rays, may serve as a shield against undesirable X- rays, though in some cases I may provide an additional shield against undesirable X-rays in the form of a casing 45. In such a case, said casing is provided in the general neighborhood of 46, with a portion substantially transparent to desirable X-rays coming within the conical space 13, so that they may emerge into the useful X-ray field, such as that below 46.

The small parts, such 14, 15, 16, 17, 18, 19, 21, 22, 23 and the like are shielded electrostatically by means of the focusing device 11, in such a way as to prevent said small parts from coming into the electrostatic field to any substantial degree and thereby influencing it. So it may be pointed out that in viewing the X-ray tube in its casing, it will be noted that the outstanding characteristic is a substantial absence of points, edges and surfaces of small radii of curvature in the construction of parts used in locations which influence the electrostatic field distribution caused by the high voltage employed in connection with my device, and I thereby not only improve the dielectric characteristics of the insulation properties utilized in my device, but I also improve the general operation of the X-ray tube itself.

It will be appreciated by those familiarwith the art, that gaseous ionization within an X- ray tube may be dependent upon the maximum voltage stress within the rarefied gas thought necessary heretofore, to produce X- ray tubes which operate independent of -troublesome gaseous ionization.

It will be appreciated that in all X-ray tube-s, regardless of how much'care is exercised in pumping out the residual gases, there is not only theoretically but practically in This may be made evident by taking at random'any hot-cathode tube of the art prior to my several inventions and with the filament cold impressing the maximum allowable potential upon the terminals of the tube and observing the phenomena which take place therewithin. Minute pale green fluorescent spots on the glass walls of said tubes will'appear and upon investigation,

these spots wiil be found to be sources'of-X- rays produced by gaseous ions developed by the high dielectric field stresses-caused by'the sharp edges of the focusing device of the said tube, bearing irrefutable evidence of gaseous ionization. Y

By the employment of my surface-characteristic means, for controlling the electrostatic field distribution, such'as for example, the spheroidal ended anode and the spheroidal toroidal focusing device, for reducing the dielectric field stresses about my elec trodes, I find that I can produce a more dependable X-ray tube with less gaseous ionization present, than in the prior art tubes and with higher gas pressures than possible without my improved electrodes, and further, with such pressures as have been common prior to the general use of thermionic X-ray tubes.

Further, it will be appreciated that X-ray tubes which are dependent upon gaseous attenuation to reduce the gas ionization within the tube, are subject to instability and erratic behavior due to minute gas pressure changes in the envelopes of said tubes. In these devices, pointed or edged electrodes do not have very pronounced working characteristics, and work more or less good or bad,

depending upon the instantaneous value of the gas pressure within the tube. However, in my device, this characteristic is not strongly present, as more sharply defined critical points are found present during its operation and phenomena due togaseous ionization may be almost non-existent up to critical values, where it becomes more pronounced with greater rapidity.

It will be appreciated that with such a characteristic, so long as the critical point every tube, appreciable gaseous ionization.

has not been overstepped, gaseous ionization v is conspicuously absent, and the X-ray tube Y may be operated with constantly changing and fluctuating gas pressures, without these changes or fluctuations being apparent in the operation or interfering with the stability of the X-ray tube.

The principal and essential elements of my 7 invention have been described and their operation recited, except for minor details which are common knowledge and immediately obvious to those skilled in the art and which are directly apparent from the dis closure.

7 Although a preferred embodiment of my invention is illustrated in detail, it is obvious that changes and modifications may be made therein without departing from the Y spirit and scope of my invention, as defined by the appended claim.

What is claimed is:

An X-ray device comprising an evacuated envelope, a target and a thermionic cathode therein, leading-in conductors for said cathode, and a focussing device for said cathode insulatingly supported by said leadingin conductors.

In testimony whereof, I have hereunto subscribed my name this 81st day of December 1923.

MONTFORD MORRISON. 

